Band gap voltage references are reference circuits widely used in integrated circuits, usually to provide a temperature-stabilized output voltage. The reference circuit uses the voltage difference between two pn-junctions operated at different current densities. The voltage difference may generate a current proportional to absolute temperature in a first resistor, which is then used in a subsequent step to generate a voltage in a second resistor. Said voltage in turn is added to the voltage of an additional pn-junction and may provide a current which is complementary to absolute temperature. If the ratio between the first and second resistor is chosen properly, the first order effects of the temperature dependency of the pn-junction and the temperature-depending current will cancel each other out.
FIG. 8 shows a known band gap reference circuit comprising a first branch with transistor M1, resistor R1 and bipolar transistor Q1 connected in series. A second branch comprises transistor M2 and bipolar transistor Q2 also connected in series. A node in the first and second branches, particularly between the transistors M1, M2 and the respective bipolar transistors Q1 and Q2 are connected to a common comparator A providing a control signal Vg controlling field-effect transistors M1, M2, respectively. The output of comparator A is also connected to transistor M5 being part of an output branch also comprising resistor R6 and bipolar transistor Q3.
In operation, the control signal Vg corresponding to the gate voltages of transistors M1, M2, and M5 controls the current through transistors M1 and M2 such that voltages V1, V2 at nodes 10 and 11, respectively, are equal. At the same time, resistor R1, a floating resistor, comprises a temperature dependency. Similarly, output resistor R6 comprises the same or bigger resistance value and therefore a stronger temperature dependency compared to resistor R1. The voltage across transistor Q3 comprises the opposite temperature dependency, for instance the complementary to absolute temperature dependency. As a result, both temperature dependencies of a voltage across resistor R6 and transistor Q3 will cancel each other, thus resulting in a constant output voltage Vref. The output voltage Vref is given by the voltages across transistor Q3 and the voltage through resistor R6.
When realizing the band gap reference according to FIG. 8 in a semiconductor material as an integrated circuit, one has to implement resistors R1, R6, both in the range of several Mohms, resulting in an increase of the space required in the semiconductor material, particularly, if low currents are required.